Prior to feeding a sheet to a sheet processing machine such as a printing press, it is desirable to position the sheet in correct registration, i.e., to properly align the front and side edges of the sheet. Generally, as sheets are initially fed in a forward direction, the front edge of the sheet is first aligned on front lays (forward abutments), followed by a procedure for accomplishing the side edge alignment. Various methods and systems have attempted to effect this side edge alignment, including systems that perform the lateral alignment along a fixed side lay, i.e., lateral abutment, or without such a side lay.
For example, U.S. Pat. No. 3,743,277 (corresponding to German Patent DE 2 046 602) discloses a device which attempts to achieve lateral sheet alignment without a side lay. After the front edge of each sheet is aligned on front lays, the side position of the sheet is detected with a measuring head, and then compared with an ideal position value to determine the difference. As a result, the amount of lateral movement necessary to laterally align the sheet is known. A transversely displaceable suction pulling strip or a drum disposed in the feed table moves the sheet laterally in accordance with the obtained difference measured.
However, because this prior system does not utilize a side lay, not only must the lateral position measurement be extremely precise, therefore requiring complex circuitry, but the side-pulling means must not even slightly skew the sheet during its operation since no lateral abutment is present to readjust sheets that have shifted during the movement.
Another sheet alignment method and device that attempts to align sheets without a side lay is disclosed in U.S. Pat. No. 4,613,125 (corresponding to German Patent DE 3 301 722). In this method, the sheet is similarly aligned entirely by measuring and without utilizing a lateral abutment, by first aligning the front edge of a sheet and then laterally moving the sheet beyond a measuring zone. When the sheet exceeds the zone, a scanning device is completely cleared, thereby triggering a signal indicating that the sheet is positioned at a known lateral location. An additional movement over a fixed distance places the sheet in registration.
Again, since there is no fixed side lay, the lateral position measurement must be extremely precise, requiring sensitive scanning means, and the side-pulling means must not even slightly skew the sheet during the movement, otherwise the angled sheet will trigger the scanner improperly and remain aligned improperly.
U.S. Pat. No. 4,264,068, (corresponding to German Patent DE 2 851 935), assigned to the present assignee, discloses a sheet alignment mechanism that includes a side lay. The sheet is moved against the side lay by a pneumatic side-pulling suction means which is disposed beneath the feed table and operates in synchronization with the incoming sheets. The pneumatic side-pulling means includes an interchangeable suction device that moves in a reciprocating fashion by being mechanically coupled to a control cam. A bleed valve controls the amount of suction pressure so that the sheet slides over the surface of the moving suction means once the sheet edge abuts the side lay. The sliding continues until a valve block with a rotary plate valve is opened, which occurs when the side-pulling means has reached a certain limit and pressure is released.
With this prior system the pneumatic pulling means operates in synchronization with the incoming sheets, and suction is applied from the moment that the suction plate is pulled. As a result, because the lateral positions of the sheets vary prior to alignment, different sheets are moved different distances before reaching the side lay, and sheets are held against the side lay until the suction pressure is released for different lengths of time. Although once the sheets reach the side lay the suction pressure is such that the sheets slide over the surface of the suction means, some inertial and frictional forces are present. Thus the sheets strike the side lay at varying speeds and are forced slightly into the side lay, which causes rebounding from the side lay once released.
When printing machine speeds are high, (for example when exceeding 12,000 sheets per hour), this frictional force along with the inertial forces of the sheets and other dynamic conditions unpredictably influence the speed that the sheet initially strikes the side lay and the distances that the various sheets rebound when they are held for varying lengths of time against the side lay. As a result, at these higher speeds sheets cannot be consistently transferred to the processing machine in perfect registration, and poor operating results occur.
Finally, since these systems reciprocate continuously, the suction means must travel an extra distance toward the lateral abutment before uncovering an inlet port to apply suction. This is necessary so as to not suck in the sheets before they aligned on the front lays, while still keeping the suction means in synchronization with the incoming sheets. Thus, the overall reciprocation speed had to be increased to account for this extra distance traveled. Accordingly, because the forces differ as a function of the speed and distance that the sheets are pulled, at particularly high speeds sheets have a tendency to sustain edge damage, particularly when certain grades of paper are used.